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FreeBSD-EN-21:22.linux_futex                                    Errata Notice
                                                          The FreeBSD Project

Topic:		Linux compatibility layer futex(2) system call vulnerability

Category:       core
Module:         kernel
Announced:      2021-06-29
Credits:        Dmitry Chagin
Affects:        All supported versions of FreeBSD.
Corrected:      2021-06-29 19:58:32 UTC (stable/13, 13.0-STABLE)
                2021-06-29 20:06:09 UTC (releng/13.0, 13.0-RELEASE-p3)
                2021-06-29 20:01:48 UTC (stable/12, 12.2-STABLE)
                2021-06-29 20:26:15 UTC (releng/12.2, 12.2-RELEASE-p9)
                2021-06-29 20:01:14 UTC (stable/11, 11.4-STABLE)
                2021-06-29 20:25:38 UTC (releng/11.4, 11.4-RELEASE-p12)
CVE Name:       CVE-2018-6927

For general information regarding FreeBSD Errata Notices and Security
Advisories, including descriptions of the fields above, security
branches, and the following sections, please visit
<URL:https://security.FreeBSD.org/>.

I.   Background

The Linux ABI layer (Linuxulator) allows Linux binaries to be executed on a
FreeBSD kernel.  This compatibility layer is supported on the amd64, aarch64
and i386 architecture.

II.  Problem Description

A programming error in the Linux compatibility layer futex(2) system
call might allow attackers to cause a denial of service.

III. Impact

It is possible for an unprivileged local attacker to specify negative
wake or requeue value for futex_requeue, which may result in a signed
integer overflow.

IV.  Workaround

No workaround is available.  Systems not using the Linux binary compatibility
layer are not affected.

The following command can be used to test if the Linux binary compatibility
layer is loaded:

# kldstat -m linuxelf && kldstat -m linux64elf

V.   Solution

Upgrade your system to a supported FreeBSD stable or release / security
branch (releng) dated after the correction date and reboot.

Perform one of the following:

1) To update your system via a binary patch:

Systems running a RELEASE version of FreeBSD on the amd64, i386, or
(on FreeBSD 13 and later) arm64 platforms can be updated via the
freebsd-update(8) utility:

# freebsd-update fetch
# freebsd-update install
# shutdown -r +10min "Rebooting for an erratum update"

2) To update your system via a source code patch:

The following patches have been verified to apply to the applicable
FreeBSD release branches.

a) Download the relevant patch from the location below, and verify the
detached PGP signature using your PGP utility.

# fetch https://security.FreeBSD.org/patches/EN-21:22/linux_futex.patch
# fetch https://security.FreeBSD.org/patches/EN-21:22/linux_futex.patch.asc
# gpg --verify linux_futex.patch.asc

b) Apply the patch.  Execute the following commands as root:

# cd /usr/src
# patch < /path/to/patch

c) Recompile your kernel as described in
<URL:https://www.FreeBSD.org/handbook/kernelconfig.html> and reboot the
system.

VI.  Correction details

This issue is corrected by the corresponding Git commit hash or Subversion
revision number in the following stable and release branches:

Branch/path                             Hash                     Revision
- -------------------------------------------------------------------------
stable/13/                              7a37d13b6cfa    stable/13-n246121
releng/13.0/                            d1fffaed2309  releng/13.0-n244751
stable/12/                                                        r370058
releng/12.2/                                                      r370064
stable/11/                                                        r370057
releng/11.4/                                                      r370061
- -------------------------------------------------------------------------

For FreeBSD 13 and later:

Run the following command to see which files were modified by a
particular commit:

# git show --stat <commit hash>

Or visit the following URL, replacing NNNNNN with the hash:

<URL:https://cgit.freebsd.org/src/commit/?id=NNNNNN>

To determine the commit count in a working tree (for comparison against
nNNNNNN in the table above), run:

# git rev-list --count --first-parent HEAD

For FreeBSD 12 and earlier:

Run the following command to see which files were modified by a particular
revision, replacing NNNNNN with the revision number:

# svn diff -cNNNNNN --summarize svn://svn.freebsd.org/base

Or visit the following URL, replacing NNNNNN with the revision number:

<URL:https://svnweb.freebsd.org/base?view=revision&revision=NNNNNN>

VII. References

The fix was modeled after Linux, where a similar error has been fixed:
<URL:https://cve.mitre.org/cgi-bin/cvename.cgi?name=CVE-2018-6927>

The latest revision of this advisory is available at
<URL:https://security.FreeBSD.org/advisories/FreeBSD-EN-21:22.linux_futex.asc>
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